Chlorure d'aluminium | EPFL Graph Search

Le chlorure d'aluminium (AlCl3) aussi appelé trichlorure d’aluminium, ou chlorure d'aluminium(III), est un sel d'aluminium et de chlore. La forme anhydre possède une structure particulière : bien qu'étant un halogénure d'un métal très électropositif, les liaisons chimiques sont principalement covalentes et non pas ioniques comme on pourrait s'y attendre. Cela a notamment pour conséquence qu'AlCl3 possède des températures de fusion et de sublimation basses ( pour cette dernière), et qu'à l'état liquide, il conduit mal l'électricité contrairement aux halogénures ioniques comme le chlorure de sodium. Ce composé existe à l'état solide sous la forme d'un réseau hexacoordiné. Il fond en formant un dimère tétracoordiné, Al2Cl6 qui peut se vaporiser, mais qui se dissocie à température plus élevée pour former une espèce AlCl3 analogue à BF3. Le chlorure d'aluminium est très déliquescent et il peut exploser au contact de l'eau sous l'effet de l'hydratation. Il s'hydrolyse partiellement en pré

A mixture model to take into account diluted gas in liquid flow: applications to aluminium electrolysis

Emile Tryphon Pierre Soutter

Aluminium is a metal sought in the industry because of its various physical properties. It is produced by an electrolysis reduction process in large cells. In these cells, a large electric current goes through the electrolytic bath and the liquid aluminium. This electric current generates electromagnetic forces that set the bath and the aluminium into motion. Moreover, large quantities of carbon dioxide gas are produced through chemical reactions in the electrolytic bath: the presence of these gases alleviates the density of the liquid bath and changes the dynamics of the flow. Accurate knowledge of this fluid flow is essential to improve the efficiency of the whole process.The purpose of this thesis is to study and approximate the interaction of carbon dioxide with the fluid flow in the aluminium electrolysis process.In the first chapter of this work, a mixture-averaged model is developed for mixtures of gas and liquid. The model is based on the conservation of mass and momentum equations of the two phases, liquid and gas. By combining these equations, a system is established that takes into account the velocity of the liquid-gas mixture, the pressure, the gas velocity and the local gas concentration as unknowns.In the second chapter, a simplified problem is studied theoretically. It is shown that under the assumption that the gas concentration is small, the problem is well-posed. Moreover, we prove a priori error estimates of a finite element approximation of this problem.In the third chapter, we compare this liquid-gas model with a water column reactor experiment. Finally, the last chapter shows that the fluid flow is changed in aluminium electrolysis cells when we take into account the density of the bath reduced by carbon dioxide. These changes are quantified as being of the order of 30% and explain partially the differences between previous models and observations from Rio Tinto Aluminium engineers.

EPFL2022

Aluminium and alkali uptake in calcium silicate hydrates (C-S-H)

Calcium silicate hydrate (C-S-H) is the main hydrate phase in Portland cements. Its composition varies depending on the Ca/Si ratio and on the presence of other ions such as aluminium (called CA- S-H) or alkalis. This thesis aimed to study the composition and the structure of C-S-H. An experimental data base was realised on the effect of different Ca/Si and Al/Si ratios, alkali concentrations (potassium and sodium hydroxide) and of different temperatures on C-S-H after equilibration times up to 1 year. The composition of the solid phases was analysed by TGA, XRD/Rietveld analysis and 29Si and 27Al MAS NMR. The solutions were analysed by ionic chromatography and pH measurements. In the absence of alkali, only C-A-S-H is formed at Al/Si ≤ 0.1. At higher Al/Si ratios, katoite, stratlingite and/or AH3 precipitate in addition to C-A-S-H, such that a part of the aluminium is consumed and limits the Al/Si ratio in C-S-H to 0.15±0.05 regardless of the Ca/Si ratios. A strong correlation between the aqueous aluminium concentration and the aluminium uptake in the solid phase is observed. Aluminium in C-S-H is observed mainly as tetrahedrally, Al(IV) or octahedrally coordinated, Al(VI) aluminium. At high Ca/Si ratios, Al(VI) is favoured and the aluminium uptake in C-S-H increases with the aqueous aluminium concentration. At low Ca/Si ratio (≤ 0.8), aluminium is observed mainly as Al(IV) and the low aqueous aluminium concentrations (below detection limit) indicate a higher affinity of aluminium towards C-S-H. The uptake of aluminium increases slightly the mean basal spacing slightly at low Ca/Si ratio and up to ≈2Å at higher Ca/Si ratios due to incorporation of aluminium and calcium in the interlayer. The presence of alkali hydroxide increases the pH leading (i) to a destabilisation of stratlingite and (ii) to higher aqueous silicon and aluminium concentrations and to lower calcium concentrations. As the aluminium uptake in C-S-H is strongly related to the aqueous aluminium concentrations, the aluminium uptake in the C-S-H increases significantly in the presence of alkali hydroxides, as aluminium is more soluble at higher pH. The increase of the temperature from 7 to 80 °C increases the phase-purity and long-range order of the C-S-H. The mean chain length remains stable up to 50°C, while at 80°C chain length increases and crosslinking is observed in the presence of aluminium. At all temperatures studied, the presence of aluminium has little effect on the calculated solubility of C(-A)-S-H. The incorporation of alkali ions in C-S-H increases with the initial concentration of alkali and with the decrease of Ca/Si ratio. The aqueous calcium concentration play a dominant role in alkali uptake, as calcium competes with the alkali ions to charge balance the negatively charged silanol groups of the C-S-H. A low concentration of calcium favours the alkali uptake in C-S-H. The uptake of alkali ions shortens the silica chain length as calcium is redistributed from the interlayer to the main sheet. The alkali uptake is not significantly influenced by the presence of aluminium in C-S-H, the hydration time or the type of cation (sodium or potassium).

EPFL2014

Interaction between liquid aluminium and solid iron Al-rich intermetallics formation

Guillaume Pasche

Intermetallic phases formation is one of the phenomena, which when coupled to thermal and mechanical strains leads to the premature wear of aluminium die casting tools made out of steel. This work focuses on two aspects of this wear problem. The first and major part investigates a simplified chemical system Fe(s)–Al(l) at 700 °C in order to study the formation of the intermetallic compounds. The second part focuses on the limitations of the tool wear by testing different coatings on the steel currently used for die casting. Experiments are performed on two sample types, differentiated by their interface orientation between the two initial phases (vertical and horizontal). Observation are also of two types. Post mortem observations are performed by light microscopy and both scanning and transmission electron microscopy. In situ observation are performed by X-ray tomography, that allows to follow the evolution of the intermetallic layer with time. In the Fe(s)–Al(l) system, one main phase, namely Fe2Al5 , forms with a particular morphology called tongue-like feature in the iron matrix. Tongue tips are generally single crystals. It is also observed that these exhibit a periodical contrast at the nanometre scale. This contrast is found to be due to a slight chemical variation within the existence domain of Fe2Al5. This splitting is linked to the spinodal decomposition of the phase observed at higher temperature. At the interface between tongues and the iron matrix, a layer of about hundred nanometres that surrounds the tongues is observed. It corresponds to the Æ2 FeAl phase. Iron matrix, due to the tongue growth, suffers a plastic deformation that is observed by recrystallisation of the direct tongue surrounding. The induced strain field also leads to porosity formation at the interface between the tongues and the matrix. Pores are then trapped in the Fe2Al5 phase and partially disappear near from the liquid interface. At this interface, a layer of the order of 10μm of Fe4Al13 is observed. The thickness of this layer is stable in the explored time range of 0-4 h. However, intermetallic blocks that detach from the layer are observed in the liquid. This is interpreted as mainly due to the dissolution of the formed layer in the bath to achieve its saturation in Fe. Intermetallic thicknesses and tongue density measurements as functions of time allow to highlight the presence of three main growth regimes. The first at the beginning of the contact corresponds to rapid emergence of the tongues in the matrix. The second is linked to their progressive slow down while they start to thicken. Both thickening and tip growth are in competition from the beginning of this regime. A third regime is however defined and corresponds to a more stable growth during which thickening and tip growth still act in parallel is but their effect is less visible as they vary with the square root of time. The tongue growth observed at the beginning of the reaction is explained by a growth front destabilisation, similarly to the destabilisation of a planar front that leads to dendritic solidification described by Mullins and Sekerka. A calculation based on their model is proposed and confirms the effect of the law aluminium diffusion in iron but is limited by assumptions made for its development. Technological investigation proceeds by modification of the studied system and comparison with the fundamental one. Two ferrous materials and one aluminium alloy are studied. In addition, different coatings deposited by electroless plating on one of the ferrous substrates are tested. A quality coefficient is proposed in order to compare the result of the contact between coating and liquid aluminium. One Co-based coating is revealed as the most promising material that allows to limit the wear of the ferrous part by liquid aluminium.

EPFL2013